This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We all have experienced pain, however, very little is known about exactly what goes on in the human body while we feel pain. By applying different combinations of painful sensations under different conditions, and measuring the responses to them, we can make indirect observations as to how the nervous system (brain, spinal cord, and nerves) processes pain. By finding out how different aspects of pain feel, we hope to get a better understanding of how humans experience pain and what new approaches we may take to treat pain. A total of 20 people will be involved as subjects for the research study. This total will include 20 normal healthy (pain-free) subjects who will participate at this research site in the Coghill Lab, Gray building, room 4087 and at the General Clinical Research Center (GCRC). All participants will be without chronic pain, not taking analgesics (pain medications) regularly and will not have taken any prescription pain medications for 2 weeks before the study. The study procedures will require 4 visits. This will include a training period to learn how the heat probe works and how to estimate the pain using the VAS. The subject will be given a series of thermal stimulations ranging from 35 degrees C to 51 degrees C on his/her non-dominant forearm during each of the visits. A single oral dose of clonidine 900 ug/placebo will be administered on visit 1 or visit 3 to help delineate the mechanism that supports the phenomenon of offset analgesia.